Various methods for functionalizing the chain-end of PEO, which is useful for capsulating water-insoluble drugs, and their applications have been studied for a long time (Harris et al, Nature Reviews Drug Discovery, 2003, Vol. 2, pages 214-221; Zalipsky et al, Bioconjugate Chemistry, 1995, Vol. 6, pages 150-165). In this respect, the processes for preparing PEO such as poly(ethylene glycol) by living anionic polymerization are well described in various literatures (e.g., Slomkowski et al, “Anionic Ring-opening Polymerization”, in Ring-Opening Polymerization: Mechanism, Catalysis, Structure, Utility, Edited by D. J. Brunelle, 1993, Chap. 3, pages 87-128; Quirk et al, “Macromonomers and Macromonomers”, in Ring-Opening Polymerization: Mechanism, Catalysis, Structure, Utility, Edited by D. J. Brunelle, 1993, Vol. 9, pages 263-293).
Further, the process for preparing block copolymers consisting of PEO and other polymers are also disclosed in various literatures (e.g., Jankova et al, Macromolecules, 1998, Vol. 31, pages 538-541; Topp et al, Macromolecules, 1997, Vol. 30, pages 8518-8520). In particular, it has been well known that a micelle prepared by using a block copolymer consisting of PEO and poly(N-isopropylacrylamide) (PNiPAM) exhibits thermo-responsive property.
On the other hand, polymeric electrolytes prepared by polymerizing vinylic monomers having a carboxylic acid, sulfonic acid, amine or ammonium group have been used as pH-responsive hydrogels (Harland et al, “Polyelectrolyte Gels, Properties, Preparation, and Applications,” ACS Symp. Series # 480, Am. Chem. Soc, 1992, Chap. 17, page 285).
However, the conventional methods are disadvantageous since they must perform a number of reaction steps. For example, the step of protecting one chain-end of PEO with a functional group such as a methyl group has to be carried out prior to the functionalization step of the other chain-end of PEO, thereby causing a low yield.